The amount of time after the last message sent in a facebook/MSN chat conversation before it's OK to log out without saying goodbye/g2g etc. It varies from person to person but usually ranges between 5-20 minutes.
Person 1: It's been 10 minutes since the last message I sent to Bob. You think that's enough chat cooldown?
Person 2: It's borderline...but Bob's a dick, so just log out.